Huge quantities of hydrocarbons are trapped in geologic formations around the world. Crude oil and natural gas are the only liquid hydrocarbons that naturally occur; they are viewed as strategic resources because of technological dependency on petroleum products including fuel and raw materials. Coal is a significant source of hydrocarbons for energy production and manufacturing.
Crude oil, natural gas, and coal are the most widely utilized sources of hydrocarbons because they are relatively inexpensive to find and refine. However, every spike in the price of common hydrocarbons stimulates interest in alternative sources of hydrocarbons.
Oil shale is such an alternative source. Oil shale is a petroleum source rock composed of inorganic sedimentary particles and appreciable organic material. Kerogen, the organic material in oil shale, is the solid precursor to crude oil, natural gas, and coal. Over geologic time, kerogen deep in the Earth decomposes under geothermal pressure and transforms into petroleum products. Some estimates suggest that global deposits of oil shale contain roughly three trillion barrels of recoverable hydrocarbons.
This geologic process can be mimicked by retorting. This method of extracting recoverable hydrocarbons involves heating oil shale to several hundred degrees centigrade in the absence of oxygen. The kerogen in the oil shale decomposes into numerous hydrocarbon-rich gases which are collected and liquefied. The liquefied “shale oil” is refined and processed similarly to crude oil.
Unfortunately, many oil shale deposits contain as little as 25% kerogen. There is a significant energy loss associated with heating the inert geologic materials in the oil shale to several hundred degrees centigrade. Thus, shale oil is not economically viable as to “energy return on energy invested” (EROEI), unless the price of crude oil is greater than about $75 a barrel. Retorting on a large scale is not desirable because of environment pollution and excessive demand upon water resources. The environmental pollution is exacerbated because potentially valuable inorganic materials and metals are not recovered from the spent shale.
The EROEI can be improved by enriching the kerogen content prior to retorting. This enrichment is very problematic because kerogen is insoluble and impervious to organic solids. One solution is to crush the oil shale in order to expose more kerogen during retorting. Other solutions include using acid treatments to reduce the amount of rock in a sample of shale.
Biotic decomposition of kerogen has been unsuccessful. There are known species of bacteria that are able to consume kerogen, but the bacteria excrete unknown compounds. On the other hand, while bacteria have been genetically-modified bacteria to excrete hydrocarbons, these do not consume kerogen.
Although producing hydrocarbons through biotic decomposition has not proven successful, there has been limited success in processing oil shale with microorganisms. For instance, the ability of certain bacteria to excrete acid has been applied to dissolve the inorganic sedimentary matrix in oil shale. (See U.S. Pat. No. 3,982,995). That inventive method produces sponge-like shale with increased surface area to promote combustion of the kerogen within. Similarly, an early inventive method used microorganisms to free kerogen in finely-ground powders of shale (See U.S. Pat. No. 2,641,565).
Coal, which is a solid derivative of kerogen, can be processed similarly to oil shale in order to obtain liquid and gaseous hydrocarbons. All methods of coal liquefaction and gasification require significant heating of the coal. This leads to the same problems as with shale oil including low EROEI and extensive environmental pollution.
Cellulose does not contain hydrocarbons. However, microorganisms such as yeast can convert cellulose and water into liquid hydrocarbons through a process called fermentation. Agricultural waste and recycled wood are common sources of cellulose for such purposes. As with methods of hydrocarbon recovery, manufacture of hydrocarbons from cellulose has significant drawbacks including low EROEI and extensive environmental pollution.